A distributed antenna system (DAS) could be used to provide radio network coverage in an enclosed area that cannot receive signals from a general mobile radio network. Non-limiting examples where DAS advantageously could be deployed are tunnels in a metro system, or a building complex.
A typical digital DAS 110 is shown in the communications network 100a of FIG. 1. Head-end equipment, hereinafter referred to as master head units (MHUs) 140 are configured to receive downlink signals from radio base stations (RBSs) in a radio access network (RAN) 150, and convert the downlink signals for transport on optical fibres in a transport network 120 to remote nodes, hereinafter referred to as remote head units (RHUs) 130, located in the coverage area of the RBSs. The RBSs are typically operated by one or more network operators. The RHUs are configured to convert the optical signals into radio signals which can be broadcast on antennas driven by the RHUs for transport to/from wireless communication devices such as smart phones and tablets. Each RHU in turn is configured to receive uplink signals from its connected antenna(s) and to convert the uplink signals for transmission over the optical fibres back to the MHUs and onward to the RBSs.
In the example of FIG. 1 the MHUs and the RBSs communicate over a radio interface. The downlink signals generated by baseband units (BBUs) 170 of the RBSs are therefore converted to radio signals by radio units (RUs) 150 in the RBSs before transmission to the MHUs. Likewise, the uplink signals are converted by the MHUs to radio signals between being transmitted to the RUs of the RBSs. The BBUs are configured to perform baseband coding and modulation in the downlink direction and demodulation and decoding in the uplink direction. Each RU is configured to convert the modulated digital baseband signals into radio signals and vice versa, using the frequency reference it has derived from the signal from its BBU.
The MHUs as well as the RHUs each have a digital interface to the DAS and in the DAS the radio signals are thus transported in the form of digital samples, usually over fibre optic connections, though not always. Digital data transport allows for flexible routing and distribution of radio signals with a much finer degree of control over which signals go to which remote nodes.
Given that the DAS is based on a digital transport, and the interface from the BBU to the RU is digital, a connection could be made directly from the BBU to an MHU in the DAS over which the digital signals can be transported directly between the BBU and the RHUs without a need for the intermediate conversion to and from analogue RF signals.
In view of the above, the DAS could thus be regarded as acting as an extension of the RBSs, which means that the signals transmitted by the RHUs must meet the same, or similar, requirements as apply to signals transmitted directly by the RBSs. One such requirement is that the frequency error (i.e., the difference between the actual and ideal transmitted channel frequency) must be small. For example, the Long Term Evolution (LTE) technical specification 3GPP TS 25.104 mandates a frequency error of between 50 and 250 parts per billion (with RBSs serving wide areas having the toughest requirement). This specification also mandates the same frequency source to be used to derive both the radio frequency and the modulation rate, so that the signal processing in the wireless communication devices only needs to compensate for a single frequency offset parameter.
Where there are multiple overlapping cells served by RBSs it is desirable that these cells use the same timing reference so that the transmit frequencies of all RBSs are the same, allowing for advanced interference management and coordinate multi-point transmissions.
The issue of frequency error becomes even more apparent in multi-operator networks (i.e., a network with multiple network operators) where RBSs from several, or at least two, different network operators are operatively connected to the same DAS. With a single network operator DAS, the DAS can be synchronized to the frequency reference of that network operator, samples can be transferred synchronously between the BBUs and the DAS, and the RHUs can generate radio signals with frequencies corresponding to the frequencies that would be generated directly by an operator RU. In a multi-operator DAS, each network operator has their own frequency reference and there is no one common frequency reference to which the DAS can be synchronized.
Hence, there is a need for improved operation of multi-operator networks where RANs of different network operators are operatively connected to the same DAS especially where the RANs are operatively connected to the DAS over digital interfaces.